1. Field of the Invention
The present invention relates to a stretched polypropylene film. More particularly, the present invention relates to a stretched polypropylene film comprising a crystalline polypropylene and a small amount of a monocyclic olefin polymer, which is significantly improved in transparency and image clarity.
2. Description of the Prior Art
Polypropylene sheets formed by extrusion are widely used in applications such as food packaging, food container and the like. Stretched polypropylene films, particularly biaxially stretched polypropylene films are widely used in packaging materials, etc. for their excellent mechanical and optical properties. They are produced generally by successive biaxial stretching using a tenter. These polypropylene sheets and stretched polypropylene films, however, are generally inferior in transparency and image clarity to highly transparent thermoplastic resins such as polystyrene, polyvinyl chloride and the like, owing to the high crystallinity of polypropylene.
Various attempts have been made in order to improve the transparency and image clarity of polypropylene sheets and stretched polypropylene films. For example, it is known that addition, to a polypropylene, of an organic or inorganic nucleating agents such as sorbitol derivative, alkali metal or aluminum salt of aromatic carboxylic acid, talc or the like makes smaller and more uniform the polypropylene spherulites and improves the transparency of stretched polypropylene film [J. Thermal Analy., 28, 161 (1983); Japanese Patent Publication No. 5935/1977; J. Appl. Polym. Sci., 9, 2131 (1965)]. These organic nucleating agents, however, showed bleeding from polypropylene during extrusion, inviting roll fouling, and gave out an offensive odor during processing. The salt of aromatic carboxylic acid reacted with other additive by itself or when hydrolyzed, deteriorating the inherent capability of the additive or inviting the coloring of polypropylene.
Talc, which is a well-known inorganic nucleating agent, showed insufficient dispersion, which allowed the polypropylene sheet or film to vary in optical properties such as transparency and the like and generated small lumps or fish eyes owing to the agglomeration of talc.
Meanwhile, it was attempted to improve the transparency of polypropylene by addition of propylene-.alpha.-olefin copolymer (GB 1104665-A corresponding to Japanese Patent Publication No. 32430/1970). In this attempt, however, no sufficient image clarity if obtained.
Thus, it has been tried to improve the transparency and image clarity of polypropylene sheet or stretched polypropylene film by the above attempts. However, no satisfactory improvement is obtained yet.
Meanwhile in U.S. Pat. No. 5,331,057 corresponding to Japanese Patent Application Kokai (Laid-Open) No. 339327/1993 is disclosed a process for producing a bock copolymer by polymerizing 0.1-95% by weight of a particular bicycloolefin, 0-95% by weight of a cycloolefin and a non-cyclic olefin in the presence of a catalyst comprising a particular metallocene. In this literature, however, no mention is made on the formation of a stretched film from said block copolymer, or on the transparency and image clarity of the stretched film.
Further in Japanese Patent Application Kokai (Laid-Open) Nos. 140222/1993, 140223/1993, 128317/1994, 128318/1994, 179709/1994 and 179717/1994 are disclosed a catalyst for olefin polymerization obtained by polymerizing a cycloolefin in the presence of a solid catalyst component (using, as an essential component, Mg and V or a mellocene compound of a transition metal) and an organoaluminum compound and contacting the resulting product with a titanium compound; and techniques similar thereto. In these literatures, however, no mention is made on the formation of a stretched film from the polymer obtained, or on the transparency and image clarity of the stretched film.